The present invention relates to a block-type tread for a pneumatic tire, and particularly, but not exclusively, to a tread for a heavy duty tire such as used on trucks, trailers or buses.
A problem occurring with block-type tire tread patterns and particularly in heavy duty tires is so-called xe2x80x98heel and toexe2x80x99 irregular wear. This occurs when the leading front portion of the tread block wears more rapidly than the rear trailing edge as the tread block passes through the contact region with the road surface, and is manifest as a saw-tooth profile to rows of blocks around the tread circumference. Whilst such irregular wear is apt to occur across the whole tread it is found to be worse in the axially outer shoulder regions.
The effect of such heel and toe irregular wear is to disfigure the pattern appearance, reduce the performance of the tread design and shorten the life of the tire.
Whilst the problem of such irregular wear may be partially mitigated by stripping the tire off the wheelrim and refitting with the reverse orientation such practice is unsatisfactory particularly in the case of a directional tread pattern which is specifically designed to be advantageously run only in one orientation.
It is known from U.S. Pat. No. 5,297,604 to provide a block-type tire tread pattern having constant and symmetrical buttressing of the blocks in the central region whilst having variable and asymmetrical buttressing to the blocks in the two axially outer median parts. According to U.S. Pat. No. 5,297,604 the values of inclination of the groove sidewalls forming the leading and trailing edges of the blocks in the outer median parts is not critical in reducing irregular wear, it is necessary only that there is asymmetry of the values.
The applicants have investigated the proposal of the prior art and found that it does not solve the problem of irregular heel and toe wear.
It is an object of the present invention to mitigate the effects of heel and toe irregular wear of a block tread pattern across the entire width of the tread and thus provide a tire having an improved block tread pattern.
According to the invention there is provided a tire having a ground contacting tread, the tread having a directional tread pattern comprising at least two substantially circumferentially extending grooves disposed one each side of the tread circumferential centerline and a plurality of circumferentially spaced apart and substantially axially extending shoulder grooves extending axially outwardly from each circumferential groove to the tread edge to define a row of tread blocks in each axially outer tread shoulder part, the axially extending grooves having two opposing inclined sidewalls which provide support to the adjacent tread block edge, characterized in that each of the axially extending shoulder grooves has a groove depth which reduces from the axially inward part to towards the axially outer part, and each groove comprises two groove main portions connected end to end, each groove main portion having a varying asymmetric cross-section along its length, each groove main portion having a first groove sidewall having a constant first angle of inclination and the opposing second groove sidewall having a varying second inclination angle, the constant first angle being greater than the maximum value of the second inclination angle, the first groove sidewall of each groove main portion being disposed on the same side of the axial groove.